Projection Lens for EUV Microlithography, Film Element and Method for Producing a Projection Lens Comprising a Film Element

Abstract

A film element of an EUV-transmitting wavefront correction device is arranged in a beam path and includes a first layer of first layer material having a first complex refractive index n.sub.1=(1−δ.sub.1)+iβ.sub.1, with a first optical layer thickness, which varies locally over the used region in accordance with a first layer thickness profile, and a second layer of second layer material having a second complex refractive index n.sub.2=(1−δ.sub.2)+iβ.sub.2, with a second optical layer thickness, which varies locally over the used region in accordance with a second layer thickness profile. The first and second layer thickness profiles differ. The deviation δ.sub.1 of the real part of the first refractive index from 1 is large relative to the absorption coefficient β.sub.1 of the first layer material and the deviation δ.sub.2 of the real part of the second refractive index from 1 is small relative to the absorption coefficient β.sub.2 of the second layer material.

Claims

1. Projection lens (PO) for imaging a pattern arranged in an object plane (OS) of the projection lens into an image plane (IS) of the projection lens with electromagnetic radiation having a working wavelength λ from the extreme ultraviolet range (EUV) comprising: a plurality of mirrors having mirror surfaces which are arranged in a projection beam path between the object plane and the image plane such that a pattern arranged in the object plane is imaged into the image plane by the mirrors, and a wavefront correction device (WFC) comprising a film element having a film which is arranged in the projection beam path in an operating mode of the wavefront correction device and is configured to transmit at the working wavelength λ a predominant proportion of the EUV radiation impinging in an optical used region, wherein the film element comprises: a first layer (L1), which consists of a first layer material having a first complex refractive index n.sub.1=(1−δ.sub.1)+iβ.sub.1 and has a first optical layer thickness, which varies locally over the used region in accordance with a first layer thickness profile; and a second layer (L2), which consists of a second layer material having a second complex refractive index n.sub.2=(1−δ.sub.2)+iβ.sub.2 and has a second optical layer thickness, which varies locally over the used region in accordance with a second layer thickness profile, wherein the first layer thickness profile differs from the second layer thickness profile, and wherein the deviation δ.sub.1 of the real part of the first refractive index from 1 is large relative to the absorption coefficient β.sub.1 of the first layer material and the deviation δ.sub.2 of the real part of the second refractive index from 1 is small relative to the absorption coefficient β.sub.2 of the second layer material.

2. Projection lens according to claim 1, wherein the film element is arranged in the projection beam path such that all rays of the projection beam are incident on the optical used region with angles of incidence of less than 20°.

3. Projection lens according to claim 1, wherein the film element has a transmittance of at least 70% for the impinging EUV radiation in an entirety of the optical used region.

4. Projection lens according to claim 1, wherein the projection lens has at least one pupil plane between the object plane and the image plane and wherein the film element is arranged in the pupil plane or optically in proximity to the pupil plane.

5. Projection lens according to claim 1, wherein at least one of: the projection lens has a film element in optical proximity to the object plane or the image plane and an intermediate image plane lies between the object plane and the image plane and a film element is arranged in the intermediate image plane or in optical proximity to the intermediate image plane.

6. Projection lens according to claim 1, wherein the film element comprises a multilayer film comprising the first layer and the second layer.

7. Projection lens according to claim 1, wherein the first layer is mounted on a first film and the second layer is mounted on a second film of the film element, said second film being physically separate from the first film.

8. Projection lens according to claim 7, wherein at least one of: a geometrical distance between the first film and the second film is less than ten centimeters an optical distance between the first film and the second film is dimensioned such that subaperature ratios of the first and second films deviate from one another by less than 0.05.

9. Projection lens according to claim 1, wherein at least one of: the working wavelength is between 5 nm and 20 nm, a first efficiency ratio V.sub.1=δ.sub.1/β.sub.1 is greater than 5, and a second efficiency ratio V.sub.2=δ.sub.2/β.sub.2 is less than 0.6.

10. Projection lens according to claim 1, wherein a first efficiency ratio V.sub.1=δ.sub.1/β.sub.1 and a second efficiency ratio V.sub.2=δ.sub.2/β.sub.2 and wherein a ratio V.sub.1/V.sub.2 is greater than 2.

11. Projection lens according to claim 1, wherein the working wavelength is in a wavelength range of 7 nm to 20 nm and wherein at least one of: the first layer material is selected from the group: ruthenium (Ru), zirconium (Zr), molybdenum (Mo), niobium (Nb), chromium (Cr), beryllium (Be), gold (Au), yttrium (Y), yttrium silicide (Y.sub.5Si.sub.3), zirconium silicide (ZrSi.sub.2), or from a material composition which predominantly consists of one of said first-layer materials, and the second layer material is selected from the group silicon (Si) and germanium (Ge) or a material composition which predominantly consists of one of said second-layer materials.

12. Projection lens according to claim 1, wherein the working wavelength is in a wavelength range of 6 nm to 7 nm and wherein at least one of: the first layer material is selected from the group: NbOB.sub.4C, NbO.sub.2, Nb.sub.2O.sub.5, RuO.sub.4, MoO.sub.2, Rh.sub.2O.sub.3, C, Te, In, Ba, Sn, RuO.sub.2, MoO.sub.3, La, B, B.sub.4C, BN, ZrO.sub.2 or from a material composition which predominantly consists of one of said first-layer materials, and the second layer material is selected from the group Y or Rb or a material composition which predominantly consists of one of said second-layer materials.

13. Projection lens according to claim 1, wherein at least one of: a first PV ratio between a largest local value and a smallest local value of the first optical layer thickness in the optical used region is between 2 and 6, and a second PV ratio between a largest local value and a smallest local value of the second optical layer thickness in the optical used region is between 2 and 6.

14. Projection lens according to claim 1, wherein the second layer thickness profile is complementary to the first layer thickness profile.

15. Projection lens according to claim 1, wherein the layer thicknesses of the first layer and of the second layer are such that the film, in a region of maximum wavefront change, brings about a wavefront change of at least 3% of the working wavelength.

16. Projection lens according to claim 1, wherein the second layer thickness is greater than the working wavelength at at least one position in the optical used region.

17. Projection lens according to claim 1, wherein the first layer has in the optical used region an asymmetrical first layer thickness profile having neither a mirror symmetry nor a radial symmetry or a rotational symmetry.

18. Projection lens according to claim 1, wherein the film has a first film surface, a second film surface and a film thickness, measured between the first and second film surfaces, of less than 1 μm, wherein the film thickness is 300 nm or less.

19. Projection lens according to claim 1, wherein the film has at at least one film surface an outer protective layer consisting of a protective layer material that is more resistant to ambient influences than is an inner layer directly adjacent to the protective layer.

20. Projection lens according to claim 1, wherein the film comprises only a single first layer, only a single second layer, or only a single first and a single second layer.

21. Projection lens according to claim 1, wherein the film element comprises a multilayer film comprising at least one antireflection layer which has a reflection-reducing effect for the working wavelength.

22. Projection lens according to claim 1, wherein at least one intermediate layer is arranged between the first layer and the second layer, and wherein the intermediate layer is at least one of: an antireflection layer and a diffusion barrier layer.

23. Projection lens according to claim 1, wherein the film element comprises a multilayer film comprising fewer than 10 further layers in addition to the first layer and the second layer.

24. Projection lens according to claim 1, wherein at least one of the first layer and the second layer is constructed with a heterogeneous layer structure, and wherein the first layer is molybdenum-based and has an inner layer structure in which relatively thick partial layers composed of molybdenum are separated by a relatively thin crystallization stop layer.

25. Projection lens according to claim 1, wherein the optical used region has a smallest diameter of at least 50 mm.

26. Projection lens according to claim 1, wherein the film element has a lattice-like supporting structure which, in the optical used region, is in contact with and stabilizes the film, and which has struts that form polygonal openings.

27. Projection lens according to claim 1, wherein the film element has a frame that supports the film to be self-supporting in the optical used region.

28. Projection lens according to claim 1, further comprising a holding structure retaining the mirrors at predetermined respective positions in the projection beam path, and wherein the film element is arranged on a changeable holder, which is movable relative to the holding structure, such that the film element is arranged optionally in the projection beam path or outside the projection beam path by movement of the changeable holder.

29. Film element, comprising: a film, which is configured to transmit, at a working wavelength λ from the extreme ultraviolet range (EUV), a predominant proportion of the EUV radiation impinging on the film element in an optical used region, wherein the film element comprises: a first layer, which consists of a first layer material having a first complex refractive index n.sub.1=(1−δ.sub.1)+iβ.sub.1 and has a first optical layer thickness, which varies locally over the used region in accordance with a first layer thickness profile; and a second layer, which consists of a second layer material having a second complex refractive index n.sub.2=(1−δ.sub.2)+iβ.sub.2 and has a second optical layer thickness, which varies locally over the used region in accordance with a second layer thickness profile, wherein the first layer thickness profile differs from the second layer thickness profile, and wherein the deviation δ.sub.1 of the real part of the first refractive index from 1 is large relative to the absorption coefficient β.sub.1 of the first layer material and the deviation δ.sub.2 of the real part of the second refractive index from 1 is small relative to the absorption coefficient β.sub.2 of the second layer material.

30. Film element according to claim 29, wherein the film element has a transmittance of between 70% and 90% for the impinging EUV radiation in the entire optical used region.

31. Projection lens (PO) for imaging a pattern arranged in an object plane (OS) of the projection lens into an image plane (IS) of the projection lens with electromagnetic radiation from the extreme ultraviolet range (EUV) around a working wavelength λ, comprising: a plurality of mirrors having mirror surfaces which are arranged in a projection beam path between the object plane and the image plane such that a pattern arranged in the object plane is imaged into the image plane by the mirrors, wherein rays of a projection beam that run between the object plane and the image plane form a wavefront, and a wavefront correction device (WFC) comprising a film element which is arranged in the projection beam path in at least one operating mode of the wavefront correction device and is configured to transmit a predominant proportion of the EUV radiation impinging on the film element in an optical used region, wherein the film element is configured to alter the wavefront such that the wavefront leading to image formation in the image plane, when the film element is present in the projection beam path, is closer to a predetermined profile of the wavefront than when the film element is absent from the projection beam path.

32. Projection lens according to claim 31, wherein the film element is arranged in the projection beam path such that all rays of the projection beam are incident on the optical used region with angles of incidence of less than 10°.

33. Projection lens according to claim 31, wherein at least one pupil plane lies between the object plane and the image plane and wherein the film element is arranged in the pupil plane or optically in proximity to the pupil plane.

34. Projection lens (PO) for imaging a pattern arranged in an object plane (OS) of the projection lens into an image plane (IS) of the projection lens with electromagnetic radiation from the extreme ultraviolet range (EUV) around a working wavelength λ, comprising: a plurality of mirrors having mirror surfaces which are arranged in a projection beam path between the object plane and the image plane such that a pattern arranged in the object plane is imaged into the image plane with the mirrors, wherein rays of a projection beam that run between the object plane and the image plane form a wavefront, a first film, and a second film, which is separate from the first film, wherein each of the films, at a working wavelength λ from the extreme ultraviolet range, transmits a predominant proportion of the EUV radiation impinging on the film in an optical used region.

35. Projection lens according to claim 34, wherein at least one pupil plane lies between the object plane and the image plane and wherein the first film and/or the second film is arranged in the pupil plane or optically in proximity to the pupil plane.

36. Method for producing a projection lens of a microlithography projection exposure apparatus comprising: mounting a plurality of mirrors at respective positions such that mirror surfaces are arranged in a projection beam path between the object plane and the image plane such that a pattern arranged in the object plane is imaged into the image plane by the mirrors, determining at least one wavefront aberration of the projection lens; calculating a location-dependent wavefront correction for an installation location from the at least one wavefront aberration of the projection lens; processing a film element to effect the wavefront correction if the film element is inserted into the projection beam path at the installation location; and installing the processed film element at the installation location.

37. Method according to claim 36, further comprising: prior to determining the at least one wavefront aberration, installing the film element at the installation location within the projection beam path, after determining the at least one wavefront aberration, removing the film element from the projection beam path, and subsequently processing the film element such that the wavefront correction is effected by the film element if the film element is inserted into the projection beam path at the installation location.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0094] FIGS. 1 and 1a show components of an EUV microlithography projection exposure apparatus in accordance with one embodiment of the invention;

[0095] FIG. 2 shows a schematic section through one embodiment of a multilayer film for wavefront correction;

[0096] FIG. 3 shows some layer materials suitable for the construction of wavefront correction films in a δ-β diagram of the complex refractive index, wherein FIG. 3A illustrates layer materials for λ=13.5 nm and FIG. 3B illustrates layer materials for λ=6.9 nm;

[0097] FIGS. 4A-4C, 5A-5C, and 6A-6C show the interplay of the optical effects of a first layer composed of Mo and a second layer composed of Si on the basis of a concrete example for layer thickness profiles;

[0098] FIG. 7 shows a schematic section through an embodiment in which the first layer and the second layer are arranged in spatially separated films situated optically in proximity to one another;

[0099] FIG. 8 shows a schematic section through an embodiment of a multilayer film in which the first layer and the second layer are arranged on opposite sides of a stable film substrate;

[0100] FIGS. 9, 9a and 9b show components of an EUV microlithography projection exposure apparatus in accordance with another embodiment of the invention; and

[0101] FIGS. 10A-10H show computational results for various profiles obtained in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0102] FIG. 1 shows optical components of an EUV microlithography projection exposure apparatus WSC in accordance with one embodiment of the invention. The EUV microlithography projection exposure apparatus serves for the exposure of a radiation-sensitive substrate W, arranged in the region of an image plane IS of a projection lens PO, with at least one image of a pattern of a reflective patterning device or mask M, said pattern being arranged in the region of an object plane OS of the projection lens.

[0103] In order to facilitate the description, a Cartesian xyz coordinate system is specified, from which the respective positional relationship of the components illustrated in the figures is evident. The projection exposure apparatus WSC is of the scanner type. The mask M and the substrate are moved synchronously in the y-direction during the operation of the projection exposure apparatus, and thereby scanned.

[0104] The apparatus is operated with the radiation of a primary radiation source RS. An illumination system ILL serves for receiving the radiation of the primary radiation source and for shaping illumination radiation directed onto the pattern. The projection lens PO serves for imaging the structure of the pattern onto a light-sensitive substrate.

[0105] The primary radiation source RS can be, inter alia, a laser plasma source or a gas discharge source or a synchrotron-based radiation source. Such radiation sources generate a radiation RAD in the EUV range, in particular having wavelengths of between 5 nm and 15 nm. In order that the illumination system and the projection lens can operate in said wavelength range, they are constructed with components that are reflective to EUV radiation.

[0106] The radiation RAD emerging from the radiation source RS is collected using a collector COL and directed into the illumination system ILL. The illumination system comprises a mixing unit MIX, a telescope optical unit TEL and a field forming mirror FFM. The illumination system shapes the radiation and thus illuminates an illumination field situated in the object plane OS of the projection lens PL or in proximity thereto. In this case, the form and size of the illumination field determine the form and size of the effectively used object field OF in the object plane OS.

[0107] A reflective reticle or some other reflective patterning device is arranged in the object plane OS during operation of the apparatus.

[0108] The mixing unit MIX substantially consists of two facet mirrors FAC1, FAC2. The first facet mirror FAC1 is arranged in a plane of the illumination system which is optically conjugate with respect to the object plane OS. Therefore, it is also designated as a field facet mirror. The second facet mirror FAC2 is arranged in a pupil plane of the illumination system that is optically conjugate with respect to a pupil plane of the projection lens. Therefore, it is also designated as a pupil facet mirror.

[0109] With the aid of the pupil facet mirror FAC2 and the imaging optical assembly which is disposed downstream in the beam path and which comprises the telescope optical unit TEL and the field forming mirror FFM operated with grazing incidence, the individual mirroring facets (individual mirrors) of the first facet mirror FAC1 are imaged into the object field.

[0110] The spatial (local) illumination intensity distribution at the field facet mirror FAC1 determines the local illumination intensity distribution in the object field. The spatial (local) illumination intensity distribution at the pupil facet mirror FAC2 determines the illumination angle intensity distribution in the objet field.

[0111] The projection lens PO serves for the reducing imaging of the pattern arranged in the object plane OS of the projection lens into the image plane IS that is optically conjugate with respect to the object plane and lies parallel thereto. The imaging is effected with electromagnetic radiation from the extreme ultraviolet range (EUV) around a working wavelength λ, which in the case of the example is 13.5 nm.

[0112] The projection lens comprises six mirrors M1 to M6 having mirror surfaces which are arranged in a projection beam path PR between the object plane OS and the image plane IS in such a way that a pattern arranged in the object plane or in the object field OF is imaged into the image plane or the image field IF via the mirrors M1 to M6. In this case, the rays of the projection beam that run between the object plane and the image plane form a wavefront WF.

[0113] The mirrors (EUV mirrors) M1 to M6 having a reflective effect for radiation from the EUV range each comprise a substrate, on which is applied a multilayer arrangement having a reflective effect for radiation from the extreme ultraviolet range and comprising a large number of layer pairs comprising alternately relatively low refractive index and relatively high refractive index layer material and acting in the manner of a distributed Bragg reflector.

[0114] The layer pairs (bilayer) comprise alternately applied layers of a layer material having a higher real part of the refractive index (also called “spacer”) and of a layer material having a lower real part of the refractive index relative thereto (also called “absorber”). Layer pairs can be constructed e.g. with the layer material combinations of molybdenum/silicon (Mo/Si) and/or ruthenium/silicon (Ru/Si). In this case, silicon respectively forms the spacer material, while Mo and/or Ru respectively serve as absorber material. A layer pair can contain at least one further layer, in particular an interposed barrier layer, which can consist e.g. of C, B.sub.4C, Si.sub.xN.sub.y, SiC or of a composition comprising one of these materials and is intended to prevent interdiffusion at the interface.

[0115] The mirrors M1 to M6 each have curved mirror surfaces, such that each of the mirrors contributes to the imaging. The rays of the projection beam path which come from the object field OF are firstly incident on the slightly convexly curved first mirror M1, which reflects the rays to the slightly concavely curved second mirror M2. The latter reflects the rays to the convex third mirror M3, which deflects the rays laterally to the concave mirror M4. The latter reflects the rays onto the fifth mirror M5, which is arranged geometrically in proximity to the image plane and which has a slightly convexly curved mirror surface and reflects the rays to the large concave mirror M6, which is the last mirror from the image plane and focuses the rays in the direction of the image field IF.

[0116] The projection lens consists of two partial lenses. In this case, the first four mirrors M1 to M4 form a first partial lens, which generates an intermediate image IMI in the ray path between the fourth mirror M4 and the fifth mirror M5. The intermediate image lies in an intermediate image plane that is optically conjugate with respect to the object plane and with respect to the image plane. Geometrically, the intermediate image is arranged alongside the sixth mirror M6. The second partial lens, which consists of the fifth and sixth mirrors, images the intermediate image onto the image plane in a reduced fashion.

[0117] Projection exposure apparatuses and projection lenses having this or a similar construction are disclosed for example in the U.S. Pat. No. 7,977,651 B2. The disclosure of said patent is incorporated by reference in the content of this description.

[0118] The projection lens PO comprises a wavefront correction device WFC, which comprises a film element FE having an optical used region UA, which is arranged in the projection beam path PR in the illustrated operating mode of the wavefront correction device. The multilayer film MF, which is partly transmissive to the EUV radiation, is arranged in the single beam path between the second mirror M2 and the third mirror M3. From an optical standpoint, it is situated between the pupil plane PS1 of the first partial lens and the intermediate image IMI optically relatively close to the pupil surface PS1. The subaperture ratio can be in the range of between 0.8 and 0.95, for example, at the location of the film element.

[0119] As illustrated in the detailed rendering of FIG. 1a, multilayer film MF is a largely planar optical element and is situated in the projection beam path such that radiation passes through it substantially perpendicularly, i.e. substantially parallel to the surface normal N of the multilayer film MF. The angles of incidence measured between the ray direction and the surface normal N are in the range of less than 10°. A polarization-selective effect is thereby avoided, such that the transmission of the multilayer film MF is substantially independent of the polarization state or of the oscillation direction of the electric field vector of the rays passing through.

[0120] The film element FE has a mechanically stable frame R, which is configured in a substantially ring-shaped manner and which supports the multilayer film MF such that the multilayer film is self-supporting in the optical used region UA. All frame elements are therefore situated outside the optical used region. The self-supporting film can be tensioned or sagging. It can have a slightly wrinkled form, if appropriate.

[0121] In other embodiments, a lattice-like supporting structure is provided for stabilizing the multilayer film in the optical used region, said supporting structure being in contact with the multilayer film in the optical used region and stabilizing said multilayer film. The lattice-like supporting structure can have, for example, a honeycomb structure having struts that form hexagonal openings. Film elements comprising such supporting structures are known from U.S. Pat. No. 7,639,418 B2, for example, and are used therein as “spectral purity filter” in the region of the EUV light source of a projection exposure apparatus.

[0122] FIG. 2 shows a schematic section through one embodiment of a multilayer film MF, which can be used in the film element FE in FIG. 1 or elsewhere. The multilayer film comprises six layers having different functions, which in some embodiments can be mechanically stabilized by an optional support structure CS at a side of the layer stack. In the installation state, radiation passes through the multilayer film substantially perpendicularly to the film plane (x-y plane). From the radiation exit side (at the bottom in the figure), the layer stack begins with a first outer protective layer PC1, to which a first antireflection layer AR1 is applied. This is followed by the first layer L1, which has a relatively small real part of the refractive index or a relatively large deviation δ.sub.1 of the real part of the refractive index from the value 1 and also a relatively low first absorption coefficient β.sub.1. A second antireflection layer AR2 is applied to the first layer. Said second antireflection layer bears a second layer L2 composed of a second layer material, which, in comparison with the first layer material, has only a relatively small deviation δ.sub.2 of the real part of the refractive index from 1, but in return has a relatively high absorption coefficient δ.sub.2. The layer stack terminates with a second outer protective layer PC2 at the radiation entrance side.

[0123] In contrast to what is shown in the schematic illustration, the layer thickness d.sub.1 of the first layer varies within the optical used region in a lateral direction, such that d.sub.1=f(x,y) holds true. The same correspondingly applies to the second layer L2. The first layer thickness d.sub.1 thus varies in the x-direction and y-direction. The second layer L2 also has a locally varying layer thickness d.sub.2, which can change locally both in the x-direction and in the y-direction. The extent of the layer thickness variations is distinctly above the extent of manufacturing-dictated layer thickness variations.

[0124] The outer protective layers PC1, PC2 can consist of ruthenium, rhodium or silicon nitride, for example, wherein silicon nitride may be advantageous owing to its low absorption at the working wavelength (13.5 nm). The first protective layer PC1 and/or the second protective layer PC2 can be omitted, if appropriate. It may sometimes suffice to have an outer protective layer by oxidation of the surface layer.

[0125] The antireflection layers AR1, AR2 here each have a geometrical layer thickness of approximately 6 nm, which corresponds to an optical layer thickness of approximately λ/2 in the case of the layer materials used (e.g. Mo/Si or Ru/Si), thus resulting in a reflection-reducing and hence in this respect transmission-increasing effect. The first antireflection layer AR1 and/or the second antireflection layer AR2 can also be omitted.

[0126] Primarily the first layer L1 and the second layer L2 are crucial for the optical effect of the film. The primary function of the first layer L1 consists in introducing in the rays passing through, in a location-dependent manner, a phase delay Δρ dependent on the local optical layer thickness of the first layer, thus resulting in locally different phase delays and hence a wavefront correction on a wavefront passing through. However, on account of the non-vanishing absorption, the first layer material also introduces a location-dependent attenuation of the radiation intensity passing through, the extent of the attenuation being greater in relatively thicker regions than in relatively thinner regions. This results in a generally unwanted location-dependent intensity attenuation effect. The primary function of the second layer L2 is to counteract the transmission attenuation introduced by the first layer in a manner such that an intensity profile required for the projection lens is established overall over the optical used region, for example a uniform attenuation over the entire used region or an attenuation having a substantially rotationally symmetrical characteristic with a rise or fall of the apodization from the center to the edge of the pupil plane. At the same time, the second layer material, on account of the relatively small deviation δ.sub.2 of the real part of the refractive index from the value 1, is intended to have only a small effect on the wavefront, which, if appropriate, can already be taken into account in the design of the layer thickness profile of the first layer.

[0127] FIG. 3A illustrates some layer materials suitable for the construction of wavefront correction films for the working wavelength λ=13.5 nm. The diagram shows the deviation δ of the real part of the complex refractive index from the value 1 on the x-axis and the absorption coefficient β on the y-axis. The materials to the left of the straight line δ=β are particularly suitable as second layer material, while the materials to the right of said straight line, in conjunction with a comparatively low real part of the refractive index, have lower absorption and are therefore particularly suitable for the wavefront correction layer (first layer). The values in the diagram are derived from a corresponding diagram in the dissertation “Surface and Interface Dynamics in Multilayered Systems” by T. Tsarfati (2009) ISBN 978-90-5335-197-0, Chapter 1, page 12.

[0128] Table A below shows the corresponding values of the effectiveness ratio V=δ/β for various layer materials that can be used particularly at a working wavelength of 13.5 nm.

TABLE-US-00001 TABLE A Mo 11.84 Y 11.51 Ru 6.66 Nb 12.75 Zr 10.92 RuSi 5.82 Si.sub.3N.sub.4 2.88 ZrSi.sub.2 6.19 Si 0.55 Ge 0.17

[0129] FIG. 3B shows a corresponding diagram for the working wavelength λ=6.9 nm. It can be discerned that, by way of example, rubidium (Rb), strontium (Sr) or yttrium (Y) are suitable as material for the second layer, while for the first layer it is possible to use, for example, NbOB.sub.4C, NbO.sub.2, Nb.sub.2O.sub.5, RuO.sub.4, MoO.sub.2, Rh.sub.2O.sub.3, C, Te, In, Ba, Sn, RuO.sub.2, MoO.sub.3, La, B, B.sub.4C, BN (boron nitride), ZrO.sub.2 or a material composition that predominantly consists of one of these materials. The values are theoretical values, obtainable e.g. via: http://henke.lbl.gov/optical_constants/getdb2.html.

[0130] The interplay of the optical effects of the first and second layers is explained below on the basis of a concrete example in association with FIGS. 4 to 6.

[0131] FIG. 4A shows an excerpt from a first layer L1 composed of molybdenum (Mo) and a second layer L2 composed of silicon (Si) applied thereto, wherein the layer thicknesses of both layers vary locally in the x-direction (normalized x-axis). Both layers are shown in each case in a partly hatched fashion and have an average thickness of 2 nm and regions having positive and negative deviations therefrom. In the molybdenum layer, an increase in the layer thickness by a maximum of 1 nm is present in the region I between x=−0.8 and x=−0.6 and a layer thickness reduction by 1 nm to a minimum of 1 nm is present in the region IV between x=0.6 and x=0.8. In the silicon layer, a local layer thickness increase by a maximum of 1 nm is present in the region II between x=−0.4 and x=−0.2 and a local layer thickness minimum having a layer thickness of only 1 nm is present in the region III between x=0.2 and x=0.4.

[0132] Both layers have both a phase-delaying effect and an intensity-attenuating effect on the EUV radiation passing through in the z-direction. However, these effects are different depending on the local layer thicknesses and the optical constants δ and β of the respective layers. The following approximately hold true: δ.sub.1=δ(Mo)=0.076, β.sub.1=13(Mo)=0.006, δ.sub.2=δ(Si)=0.001 and β.sub.2=β(Si)=0.002.

[0133] Firstly, with reference to FIG. 5, only the molybdenum layer (first layer) will be considered, the layer thickness profile of which is illustrated again in FIG. 5A. FIG. 5B shows the wavefront effect Δρ.sub.1 of the first layer (molybdenum layer) in nanometers, and FIG. 5C shows the transmission-reducing effect of the first layer, that is to say the relative transmission loss. For calculating the wavefront effect, the real part (1−δ) of the refractive index or the deviation δ.sub.1 is crucial, wherein the value δ.sub.1=0.08 means that the phase velocity of the wavefront passing through is reduced from the value 1 to the value 0.92. The phase delay brought about overall as a result is linearly dependent on the local layer thickness d.sub.1. In the region I, the local layer thickness (3 nm) is 1 nm above the average layer thickness, such that here 1 nm more of the first layer material has a phase-delaying effect. A corresponding phase delay relative to the average phase delay can be discerned in FIG. 5B. The conditions are reversed in the region IV, since here only 1 nm of molybdenum has an effect in the region of the layer thickness minimum. Accordingly, a smaller phase delay results in comparison with the average phase delay (caused by 2 nm of Mo).

[0134] The extent of the intensity attenuation ΔI is also dependent on the layer thickness d. The following generally holds true:

ΔI=1−e.sup.−((4π/λ)dβ)

[0135] In this case, the layer thickness d is in the exponent of the exponential function. In the region I, a particularly high relative transmission loss arises on account of the local thickness maximum, while the smallest relative transmission loss occurs in the region IV on account of the local layer thickness minimum of the first layer thickness.

[0136] The layer thickness profile of the second layer (silicon layer) is illustrated in FIG. 6A. The layer thickness here has a somewhat more complex profile since it is particularly small (a minimum of only 1 nm) for example in the region I on account of the local layer thickness maximum of the molybdenum layer and in the region III on account of the local minimum in the Si layer, while it assumes a local maximum in each case in the regions II and IV.

[0137] FIG. 6B shows the corresponding wavefront effect Δρ.sub.2 of the second layer (Si layer) in nanometers, while FIG. 6C shows the relative transmission loss ΔI.sub.2 of the second layer as a function of the location on the x-axis.

[0138] The effects of both layers add up positionally correctly when a wavefront passes through. The wavefront effect of the multilayer film comprising a first layer composed of Mo and a second layer composed of Si is illustrated in FIG. 4B. FIG. 4C correspondingly shows the location dependence of the relative transmission loss of the multilayer film.

[0139] FIG. 4B shows the effect on the wavefront of both layers with identical scaling on the axis of the phase delay Δρ. It can be discerned that molybdenum, exhibiting a significantly greater phase delay, dominates the profile of the wavefront effect in the first region I and in the fourth region IV. In comparison therewith, the phase delay effect in the regions II and III, where a particularly large (region II) and a particularly small (region III) absolute layer thickness of the silicon layer are present, is only very small.

[0140] In the case of the total effect on the transmission (FIG. 4C) it can be discerned that the absolute extent of the intensity attenuation caused by the molybdenum layer is made more uniform by the silicon layer. The difference between maximum and minimum local intensity loss is smaller than in the case of the pure Mo layer on account of the compensating effect of the Si layer.

[0141] It is not necessary for the first layer and the second layer to be present at the same film. FIG. 7 shows by way of example a schematic section through one embodiment of a film element FE, in which the first layer and the second layer are arranged in spatially separated films situated optically in proximity to one another. A first film F1 has a thin film substrate or a thin film supporting layer SUB1, on which a first layer L1 (e.g. composed of molybdenum) having a locally varying layer thickness is applied. The first film is held by a mechanically stable first frame R1, the frame parts of which all lie outside the optical used region UA. The first frame R1 is connected fixedly, but in a releasable manner, to an identical second frame R2 using screws or in some other way. The second frame supports a second film F2. The second film F2 has a thin film substrate (film supporting layer) SUB2, on which a second layer L2 (e.g. composed of silicon) having a locally varying layer thickness is applied. The geometrical distance between the films perpendicular to the film planes is a few millimeters, e.g. between 1 mm and 10 mm. As a result, in the installed state, they are arranged practically at the same location (substantially identical subaperture ratio) of the projection beam passing through. The layer thickness profiles of the two layers L1, L2 are complementary. The mechanical stability of the two films is substantially determined by the stability of the film substrates or film supporting layers. The latter can consist e.g. of silicon or of a polymeric material.

[0142] The film substrates or film supporting layers of uniform thickness are designed for optimum transmission in conjunction with sufficient stability and have e.g. thicknesses in the range of 10 to 100 nm, preferably 20 nm to 50 nm. The material of the film layer L1 of the film F1 and L2 of the film F2, respectively, can be identical to the material of the film supporting layers SUB1 and SUB2, respectively, and even during the protection process with identical material it is not possible to distinguish between producing the film layer (L1 or L2) and the associated film supporting layer (SUB1 or SUB2).

[0143] A cleaning apparatus (not illustrated) can be mounted between the two films F1 and F2 and the frames R1 and R2, said cleaning apparatus purging the interspace between the films F1 and F2 with purging gas, for example hydrogen, at certain time intervals.

[0144] In this embodiment, both layers (the first layer and the second layer) are freely accessible for subsequent processing (e.g. with an ion beam), as a result of which the optimization of the layer thickness profiles is simplified.

[0145] FIG. 8 shows by way of example a schematic section through one embodiment of a film element FE comprising a multilayer film MF, in which the first layer and the second layer are arranged on opposite sides of a film supporting layer SUB. As a result, both layers, independently of one another, are freely accessible for processing and subsequent layer thickness profile change. The film supporting layer SUB can, for example, consist of Mo or Si or be of a multilayer composed of Mo and Si. Here, too, the material of the first and second layers L1 and L2, respectively, can be identical to that of the film supporting layer SUB and during the production process there is no need to distinguish between applying the layers L1 and L2 and the film supporting layer SUB.

[0146] FIG. 7 and FIG. 8 do not illustrate the possible option of dispensing with the film supporting layers SUB and SUB1 and SUB2 on account of sufficient stability of the first and second layers L1 and L2, respectively.

[0147] In the case of the embodiments in FIGS. 7 and 8, in principle the same conditions as presented in detail above are applicable to the selection of the layer materials. Therefore, reference is made to the description there.

[0148] FIG. 9 shows an example of another projection lens PO which is equipped or can be equipped with a film-based wavefront correction device. Elements identical or corresponding to those in FIG. 1 bear the same designations. The construction of the projection lens including the optical data is described in US 2008/0170310 A1 corresponding to EP 1 950 594 A1 (FIG. 2). The content of said documents is in this respect incorporated by reference in the content of this description.

[0149] The illustration shows the beam path of in each case three individual rays that proceed from five spaced-apart object field points.

[0150] Proceeding from the object plane OS, the individual rays are reflected firstly by a first mirror M1 and then successively by the second to sixth mirrors M2 to M6, which are in each case covered with highly reflective multilayer coatings.

[0151] The mirrors, M1, M3 and M5 have a convex basic form, that is to say can be described by a convex best-matched surface. The mirrors M2, M4 and M6 have a concave basic form, that is to say can be described by a concave best-matched surface. In the following description, such mirrors are designated merely as convex or concave for simplification. The convex third mirror M3 provides for a good Petzval correction.

[0152] The individual rays associated with a specific illumination direction of the five object field points are combined in a pupil plane PS1 of the projection lens PO, adjacent to which the third mirror M3 is arranged. The third mirror M3 is therefore also designated as a pupil mirror. An aperture stop for delimiting the projection beam can be arranged in the pupil plane PS1. Said aperture stop can be provided by a mechanical and exchangeable stop or else in the form of a corresponding coating directly on the mirror M3.

[0153] The mirrors M1 to M4 image the object plane OS into an intermediate image plane IIS. The intermediate-image-side numerical aperture of the projection lens is 0.2. The mirrors M1 to M4 form a first partial imaging optical unit having a reducing imaging scale of 3.2×. The downstream mirrors M5 and M6 form a further partial imaging optical unit of the projection lens having a reducing imaging scale of 2.5×. In the region of the intermediate image plane IIS, a through-opening OP6 is formed in the sixth mirror M6, the projection beam passing through said opening upon reflection from the fourth mirror M4 toward the fifth mirror M5. The fifth mirror M5 in turn has a central through-opening OP5, through which the projection beam passes between the sixth mirror M6 and the image plane IS.

[0154] The fifth mirror M5 is arranged in proximity to a further pupil plane PS2, which is conjugate with respect to the first pupil plane PS1. Typically, the further pupil plane PS2 is situated in the projection beam path between the fifth mirror M5 and the sixth mirror M6, such that a physically accessible stop plane exists at the location of the further pupil plane PS2.

[0155] The projection lens has an obscuration stop arranged in a centered manner in one of the pupil planes PS1, PS2. This obscures the partial rays of the projection beam path that are assigned to the central through-openings OP5, OP6. Therefore, the design of the projection lens is also designated as a design with central pupil obscuration.

[0156] A distinguished individual ray that links a central object field point to a centrally illuminated point in the entrance pupil of the projection lens in the entrance pupil plane PS1 is also designated hereinafter as chief ray CR of a central field point. Following reflection at the sixth mirror M6, the chief ray CR of the central field point forms approximately a right angle with the image plane IS, that is to say runs approximately parallel to the z-axis of the projection exposure apparatus. The image field is rectangular.

[0157] All six mirrors M1 to M6 of the projection lens PO are embodied as freeform surfaces that cannot be described by a rotationally symmetrical function.

[0158] The projection lens PO affords a plurality of positions for inserting a film element of a wavefront correction device.

[0159] In one embodiment, illustrated in plan view in FIG. 9a, a first film element FE1 in the form of a multilayer film is arranged at a first position POS1 optically in proximity to the first pupil plane PS1 in the beam path between the second and third mirrors. Wavefront aberrations can thus be corrected uniformly over the entire field. The subaperture ratio SV is in this case approximately 0.7 to 0.95.

[0160] Another embodiment, illustrated in plan view in FIG. 9b, seeks to correct a field profile of a wavefront aberration using a film element. For this purpose, a correspondingly adapted film element, e.g. a second film element FE2 in the form of a multilayer film, is inserted at a second position POS2 in optical proximity to the object plane OS between the object plane and the first mirror M1. The subaperture ratio SV is in this case approximately 0.05 to 0.3.

[0161] By way of example, the film element can be designed such that a linear or nonlinear profile of image aberrations perpendicular to the scanning direction (y-direction), i.e. along the long axis of the image field, can be corrected or reduced in its extent.

[0162] It is also possible for a film element to be arranged both at a position in proximity to the pupil and at a position in proximity to the field. This variant is shown in FIG. 9.

[0163] In order to illustrate the positions in the projection beam path, FIG. 9 illustrates the first film element FE1 the second film element also in plan view parallel to the z-direction. The circular frame R can be discerned in each case, said frame supporting the partly transparent, self-supporting multilayer film MF. The area of intersection of the projection beam with the film surface is the so-called “footprint” FP1 and FP2, respectively. This region is illuminated by the projection beam, wherein all beams proceeding from the (infinite number of) field points of the rectangular object field OF contribute to the illumination of the footprint. It can be discerned that in proximity to the object plane OS the footprint FP1 has approximately the (in the real system slotted) rectangular shape of the object field, the corner regions being rounded. In proximity to the pupil plane PS1, the footprint FP2 is virtually circular. A minimal circle enclosing the footprint shall have the diameter D.sub.CA. This diameter is designated here as the optically free diameter.

[0164] Each object field point is the origin of a beam (cone of radiation) whose aperture angle is determined by the object-side numerical aperture. Each beam is associated with a subaperture SA corresponding to that region which is illuminated on an optical surface (here: film surface) by a beam proceeding from a single object field point. A subaperture on a given surface can be characterized by its subaperture diameter D.sub.SA. In proximity to the object plane (or a different field plane), said subaperture or its diameter is relatively small. In the region of a pupil plane, ideally all subapertures of the different field points should overlap, each beam illuminating the entire used pupil.

[0165] It is qualitatively discernible that the subaperture ratio SV=D.sub.SA/D.sub.CA of the first film element FE1 in proximity to the field is relatively small (e.g. between approximately 0.05 and 0.3), while SV at the second film element FE2 arranged in optical proximity to the pupil plane PS1 is close to the value 1, e.g. between 0.7 and 0.95.

[0166] The way in which a film element can be designed in practice is explained by way of example below. The presentation applies to all embodiments.

[0167] The task of the film element is to set wavefront variation and/or transmission variation of the projection lens according to the target stipulations.

[0168] The starting point for this is wavefronts and/or transmissions at one or a plurality of field points. These can be obtained by measurements and/or simulations. An extrapolation and/or interpolation to a plurality of field points is additionally possible. These data obtained in this way are the starting point for the optimization step described below, and are designated as wavefront data and/or transmission data, respectively.

[0169] In a first step, it is assumed here that the wavefront can be corrected by a “perfect wavefront correction layer” of the film element. A “perfect wavefront correction layer” is understood to be a theoretical layer whose complex refractive index n at the working wavelength λ is n=0+0i, such that 1 nm wavefront correction layer reduction (layer thickness difference) is thus translated into 1 nm wavefront phase. The concept of a light ray modeled as moving infinitely fast is borrowed from the Sweatt model. As an illustrative alternative it is also possible to use a theoretical layer whose complex refractive index n at the working wavelength λ is n=0.9+0i, such that 1 nm wavefront correction layer reduction (layer thickness difference) is translated into 0.1 nm wavefront phase. In this case, after the calculation of the wavefront correction layer thickness, the latter has to be multiplied by the factor 0.1 in order to obtain the “perfect wavefront correction layer”.

[0170] For given wavefront data, for the film element position defined in the beam path of the projection lens, a perfect wavefront correction layer is now calculated by virtue of a suitably formulated optimization problem. For this purpose, firstly so-called basis deformations are calculated. These basis deformations can have for example the form of Zernike polynomials having a certain maximum amplitude (for example 1 nm) which are defined on a circular region that fully encompasses the optically used region (the optical used region) of the film of the film element positioned in the beam path. Besides Zernike polynomials it is also possible to use splines or B-splines or else nurbs, the computational field of which likewise fully encompasses the optically used region of the film of the film element. For these basis deformations thus obtained (for example 36 or 64 or 100 Zernikes and/or 25 or 49 or 100 splines or B-splines or nurbs), the optical sensitivity thereof is calculated with the aid of an optical design program. That is to say that the wavefront effect of the basis deformations in a perfect wavefront correction layer is calculated.

[0171] The basis deformations are then interpreted as manipulator degrees of freedom. The optimization problem then consists in approximating the wanted (field point by field point) wavefront effect as well as possible with said degrees of freedom. This can be done for example by solving the minimization problem

min∥Mx−p∥.sub.2.sup.2+∥Gx∥.sub.2.sup.2

[0172] In this case, M denotes an n×m matrix having the m basis deformations developed into n elementary image aberrations. These elementary image aberrations can be for example pixel by pixel wavefront values at different field points, selected Zernike coefficients of these wavefronts at different field points or superpositions thereof. The vector p describes the wavefront data in the predefined manner, x denotes the manipulated vector to be found, which describes the amplitudes of the basis deformations to be superposed, and G is a suitable weight matrix, for example the unit matrix provided with a scalar multiple. The method presented here is the so-called Tikhonov regularization, which is described in greater detail for example in A. Rieder, Keine Probleme mit inversen Problemen [No problems with inverse problems], Vieweg, 2003 on page 70 (example 3.3.11) and in chapter 4. That also explains how the minimization problem can be transformed into a system of equations in order to solve this by one of the known methods, such as, for example, the Gaussian elimination method. Alternatively, it is also possible to consult the internet page http://en.wikipedia.org/wiki/Tikhonov_regularization (viewed on 08.02.2012)

[0173] A further possible method is described in WO 2010/034674 A1 in connection with a different problem.

[0174] The use of such a method results in a stipulation of the profile of a “perfect wavefront correction layer” of the film element, characterized by the function w:=w(x,y) for describing the location-dependent layer thickness. The function w can have both positive and negative layer thickness values. The way in which the negative layer thickness values can be eliminated is described further below.

[0175] It should be mentioned that the procedure described above is only one exemplary procedure for calculating a perfect wavefront correction layer.

[0176] If only the transmission behavior of the projection lens is intended to be corrected, then the wavefront correction layer can be described by the function w=w(x,y)=0.

[0177] In a second step, it is firstly assumed that the transmission profile can be corrected by a “perfect transmission correction layer” of the film element.

[0178] A material having a complex refractive index n=(1−δ)+iβ at the working wavelength λ shall initially be presented. If a light ray covers a path length d in this material, then the transmission t is

t=exp(−(4π/λ)dβ).

[0179] What is disadvantageous here is that the transmission no longer varies linearly with the thickness of the material, but rather exponentially. This has the effect that firstly the approach of reducing the problem to a linear equation system fails.

[0180] By contrast, if the logarithmic transmission ln t is considered, given by

ln t=−(4π/λ)dβ,

then it is evident that the logarithmic transmission varies linearly with the thickness of the material.

[0181] A “perfect logarithmic transmission correction layer” is understood to be a theoretical layer whose complex refractive index n=0+1i.

[0182] For given transmission data T(x,y), the logarithmic transmission data ln T(x,y) are calculated. This is always possible since the transmission T(x,y) at every point (x,y) is greater than 0. For the film element position defined in the beam path of the projection lens, a perfect logarithmic transmission correction layer is calculated by virtue of a suitably formulated optimization problem.

[0183] An optimization problem is then solved. For this purpose, firstly so-called basis deformations are once again calculated. These basis deformations can have for example the form of Zernike polynomials having a certain maximum amplitude (for example 1 nm) which are defined on a circular region that fully encompasses the optically used region of the film of the film element positioned in the beam path. Besides Zernike polynomials it is also possible to use splines or B-splines or else nurbs, the computational field of which likewise fully encompasses the optically used region of the film of the film element. For these basis deformations thus obtained (for example 36 or 64 or 100 Zernikes and/or 25 or 49 or 100 splines or B-splines or nurbs), the optical sensitivity thereof is calculated with the aid of an optical design program. That is to say that the transmission effect of the basis deformations in a perfect transmission correction layer is calculated and the logarithmic transmission effect is then determined.

[0184] The basis deformations are then interpreted as manipulator degrees of freedom. The optimization problem then consists in approximating the wanted (field point by field point) wavefront effect as well as possible with said degrees of freedom. This can be done for example by solving the minimization problem

min∥Ny−q∥.sub.2.sup.2+∥Hy∥.sub.2.sup.2

[0185] In this case, N denotes a k×l matrix having the l basis deformations developed into k elementary image aberrations. These elementary image aberrations can be for example pixel by pixel transmission front values at different field points, selected Zernike coefficients of these transmission fronts at different field points or superpositions thereof. The vector q describes the logarithmic transmission data in the predefined manner, y denotes the manipulated vector to be found, which describes the amplitudes of the basis deformations to be superposed, and H is a suitable weight matrix, for example the unit matrix provided with a scalar multiple. The resulting minimization problem is solved as in the case of the wavefront data.

[0186] The use of such a method results in a stipulation of the profile of a perfect logarithmic transmission correction layer of the film element, characterized by the function s:=s(x,y) for describing the location-dependent logarithmic layer thickness.

[0187] It should be mentioned that the procedure described above is only one exemplary procedure for calculating a perfect logarithmic transmission correction layer.

[0188] If only the wavefront behavior of the projection lens is intended to be corrected, then the logarithmic transmission correction layer can be described by the function s=s(x,y)=0.

[0189] All information is now present for making the transition to real materials. Thus, let there be a material M.sub.1 for the first layer having a complex refractive index

n.sub.1=(1−δ.sub.1)+iβ.sub.1

and a material M.sub.2 for the second layer having a complex refractive index

n.sub.2=(1−δ.sub.2)+iβ.sub.2.

[0190] By way of example, molybdenum (Mo) can be used as material of the first layer and silicon (Si) can be used as material of the second layer. The material thicknesses m.sub.1=m.sub.1(x,y) and m.sub.2=m.sub.2(x, y) of the first and second materials, i.e. the first and second layer thicknesses, are thus intended to be determined such that the equation system

δ.sub.1m.sub.1(x,y)+δ.sub.2m.sub.2(x,y)=w(x,y)

exp(−4π/λ(β.sub.1m.sub.1(x,y)+β.sub.2m.sub.2(x,y)))=exp(s(x,y))

is fulfilled at every point (x,y). Suitable discretization of the points (x,y), for example on a lattice of dimension 101×101 or 201×201 or else 501×501, yields the equation system to be solved and thus information about the material thicknesses m.sub.1 and m.sub.2 to be estimated on a sufficiently fine grid. By logarithmizing the second equation, this equation system can even be reduced to a linear equation system:

δ.sub.1m.sub.1(x,y)+δ.sub.2m.sub.2(x,y)=w(x,y)

−4π/λ(β.sub.1m.sub.1(x,y)+β.sub.2m.sub.2(x,y))=s(x,y).

[0191] This equation system can be solved by the customary methods such as, for example, the Gaussian elimination method for every point (x,y) of the discretization grid and the local material thicknesses m.sub.1=m.sub.1(x,y) and m.sub.2=m.sub.2(x,y) are obtained.

[0192] In this case, the functions m.sub.1=m.sub.1(x,y) and m.sub.2=m.sub.2(x,y) can have both positive and negative function values and it is necessary to generate thickness stipulations that can be realized from these theoretical material thicknesses. Firstly, material minimum thicknesses d.sub.1(x,y) and d.sub.2(x,y) of the materials M.sub.1 and M.sub.2 are defined, which must be exceeded by a film element realized. The material minimum thicknesses can vary locally or else be constant.

[0193] In the case where Mo is used as material for the first layer, for example a location-independent minimum thickness of 5 nm or 10 nm or 20 nm can be chosen. In the case where Si is used as material for the second layer, for example a location-independent minimum thickness of 10 nm or 20 nm or 50 nm can be chosen.

the Material Thickness Stipulations

[0194]
{tilde over (m)}.sub.1(x,y)=m.sub.1(x,y)+d.sub.1(x,y)−min.sub.(x,y)m.sub.1(x,y)

and

{tilde over (m)}.sub.2(x,y)=m.sub.2(x,y)+d.sub.2(x,y)−min.sub.(x,y)m.sub.2(x,y)

are then calculated.

[0195] It is evident that the wavefront correction, which is only a matter of changing the phase effect, is realized in its full scope. However, the transmission correction is only realized up to a constant factor of less than 1, since any layer thickness at the working wavelengths considered leads to an appreciable transmission loss and the transmission of the system can only be decreased by adding material thicknesses.

[0196] FIG. 10 shows, on the basis of a numerical example, significant results obtained when using the procedure described above: FIG. 10A illustrates the profile d.sub.1′ [nm] of a “perfect wavefront correction layer” which is intended to be realized on a film element comprising a multilayer. The PV value of the difference between highest elevation and deepest valley of the “perfect wavefront correction layer” w(x,y) is approximately 1.4 nm. A wavefront profile with a maximum phase difference of approximately 1.4 nm is therefore corrected. It is additionally assumed that the film element is only intended to correct the transmission variation induced by the wavefront correction layer. The “perfect logarithmic transmission correction layer” s(x,y) can therefore be assumed to be equal to 0 as a constant.

[0197] Molybdenum (Mo) is chosen as material of the wavefront correction layer and silicon (Si) is chosen as material for the transmission correction layer. Both materials contribute to the wavefront correction. FIG. 10B shows the computational layer thickness profile d.sub.1′ [nm] of the wavefront correction layer obtained by solving the equation system. In this case, it is noticeable that both positive and negative layer thickness values occur.

[0198] The computational layer thickness values from FIG. 10B have been converted into implementable layer thickness values d.sub.1 [nm] in FIG. 10C. For this purpose, a layer having a constant layer thickness has been added to the computational layer thickness profile, such that the smallest value of the layer thickness thus obtained is greater than or equal to the minimum layer thickness specified for this material. In FIG. 10C, by way of example, the value 5 nm was chosen as a specified minimum layer thickness for molybdenum (Mo). This procedure is possible since a constant layer having the thickness of 20 nm or else 100 nm or else 500 nm does not influence the wavefront profile significantly in this context.

[0199] FIG. 10D shows the computational layer thickness profile d.sub.2′ [nm] of the transmission correction layer obtained by solving the equation system. Positive and also negative layer thicknesses once again occur in this case. If, by way of example, 20 nm is chosen as value for the specified minimum layer thickness of silicon, then this results in the implementable layer thickness profile d.sub.2 [nm] of the transmission correction layer illustrated in FIG. 10E.

[0200] The thickness profile of the multilayer of the film element is shown in FIG. 10F: an upper, rather thicker, varying layer of silicon is applied on a lower, rather thinner, varying layer of molybdenum. The complementary behavior of the thicknesses of the two layers is readily discernible at the locations x=0.75 and x=0.5. The two layers can also be interchanged, that is to say that the thinner layer of molybdenum can also be applied on the thicker layer of silicon.

[0201] FIG. 10G shows the deviation ΔWF of the wavefront profile predefined in FIG. 10A from that wavefront profile which was produced using the multilayer film of a film element described in FIG. 10F. The deviation is constant and thus optically neutral.

[0202] FIG. 10H illustrates the transmission profile of the multilayer of the film element described with reference to FIG. 10F. This transmission profile, as predefined initially, has no variation. The transmission T of approximately 83.6% illustrated here takes account only of the two layers illustrated in FIG. 10F. As already explained, a film element can comprise even further layers or else supporting structures which can additionally decrease the transmission.